Two peptidic transition-state analogs, corresponding to segments in the gp120 sequence, have been synthesized and used as immunogens for the production of murine monoclonal antibodies. Studies using supernatants from clones secreting antibody, incubated for varying lengths of time with transition-state analogs and native peptides, suggest that cleavage of the latter does take place. Several of those clones are being expanded. The secreted antibodies have been isotyped and are being tested for proteolytic activity against whole gp120 or gp160.